In a stock market, a transaction between a buyer and seller of a security is executed when the purchase price matches the selling price. In one example, such as the Nasdaq Stock Market, the orders are received from different market participants (such as Market Makers and Electronic Commerce Networks) by central servers. New incoming orders are compared against contra side orders in an in memory order book, and if a match is found (i.e., a buy order matches a sell order, or vice versa), a transaction is executed. If no match is found, the new order is stored in the in memory order book to await matching with later orders.